diff --git a/app.js b/app.js
index ad9a93a..15ca592 100644
--- a/app.js
+++ b/app.js
@@ -1 +1,46 @@
 'use strict';
+const fs = require('node:fs');
+const readline = require('node:readline');
+
+const rs = fs.createReadStream('./popu-pref.csv');
+const rl = readline.createInterface({ input: rs });
+
+const prefectureDataMap = new Map();
+
+rl.on('line', lineString => {
+  const columns = lineString.split(',');
+  const year = parseInt(columns[0]);
+  const prefecture = columns[1];
+  const popu = parseInt(columns[3]);
+
+  if (year === 2016 || year === 2021) {
+    let value = null;
+
+    if (prefectureDataMap.has(prefecture)) {
+      value = prefectureDataMap.get(prefecture);
+    } else {
+      value = {
+        before: 0,
+        after: 0,
+        change: null
+      };
+    }
+
+    if (year === 2016) {
+      value.before = popu;
+    }
+    if (year === 2021) {
+      value.after = popu;
+    }
+    
+    prefectureDataMap.set(prefecture, value);
+  }
+});
+
+rl.on('close', () => {
+  for (const [prefecture, value] of prefectureDataMap) {
+    value.change = value.after - value.before;
+  }
+  
+  console.log(prefectureDataMap);
+});
\ No newline at end of file